ADS1298 dynamic range of differential input

Jan Knezik

Other Parts Discussed in Thread: ADS1298

Hello,

I have a few questions about diferential input of ADS1298 circuit. On page 26 of the datasheet there is formula (3) that dynamic range equals 2Vref / Gain. Does it mean that for 4V reference and Gain = 1 I can have total dynamic range of 8V? If it is true what way should I connect input pre-amp to achieve this (for example AD8231 with differential output)?

Thank you,

Jan Knezik

over 15 years ago

Kaspars over 15 years ago

Intellectual 880 points

Hi Jan,

Yes you are correct, if you have configuration like this, then dynamic range will be 8V.

Here is some example circuit how you can connect your pre-amp to ADS1298 with differential input:

Hope this will help you.

Best regards,

Kaspars

Jan Knezik over 15 years ago in reply to Kaspars

Prodigy 30 points

Hi Kaspars,

thank you very much for your reply. I guess the op-amp in the middle is driving DRL (Driven Right Leg) and the aplifiers

for each line (INP, INN) are ordinary low noise and drift operation amplifiers. Together with the PGA inside ADS1298 it makes

one instrumentation amplifier. Am I right? In my opinion it could be a litlle bit tricky to achieve good CMRR. What's your opinion?

I am thinking about the DRL. Can the built-in DRL in ADS1298 be used instead of the one on the example circuit? Which way is the better?

Best regards,

Jan

Kaspars over 15 years ago in reply to Jan Knezik

Intellectual 880 points

Hi Jan,

Sorry for my late reply. About RLD amplifier and correct connection, please see this schematic:

http://openeeg.sourceforge.net/doc/modeeg/modEEGamp-v1.0.png

Like you can see its design is very close to that I give in my last post. You can use this schematic to make pre-amp stage before ADS1298. RLD is provided by IC204B and IC201A for first channel. Only thing you must change is NOT to use INA114P (IC202), but place it with circuit whitch I posted in my last post.

And Yes, ofcourse, you must use low noise/drift opamps and you also correct, that PGA and Opamps before ADS1298 make instrumental amplifier.

About CMRR, in ADS1298 it's said that CMRR = -115db, but I think, if you use the RLD like in circuit above, you could reach CMRR = -140dB.

Ofcourse you can use ADS1298 internal RLD, but I think if you will use extrnal RLD it would be better.

Best regards,

Kaspars

Jan Knezik over 15 years ago in reply to Kaspars

Prodigy 30 points

Hi Kaspars,

you are right, but in my opinion there are problems with asymmetry (op-amps with a little bit different params, resistors with tolerance) that causes this circuit not to be as good as integrated instrumentation amplifier which is laser trimmed. If the common voltage is "computed" not so good then the actual CMRR (even with DRL) of the device goes wrong.

I see the way in using ADS1298 with single-ended input and integrated instrumentation amplifier as a pre-amp.

Best regards,

Jan

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ADS1298 dynamic range of differential input